My hard drive says it’s full, but it’s not - Tech Support Forum
Hi
I am having similar problem to the link above.
Server 2003 R2 standard Edition has two separate hdd. Primary one is C (OS) and other is D for data.
C is 80 GB and it shows “free space left” 100 MB !!!
However, all of the files on C:\ add up only to 10 GB
What troubleshoot should I consider to find out why this happens
Thanks
so, you ran a virus check, right? 
anyway, what does it say when you go to C: > Properties (capacity/used space/frees pace)?
maybe it’s hidden files or something. Try ccleaner.
I also recommend cc cleaner. I used to have similar problems from time to time until I discovered this program it was amazing. This application is among the most recommended valid free registry cleaner. It has kept my computer in good running condition over the years. I’m glad that it has recently introduced its latest version upgrade to accommodate Windows 7 operating system.
Maybe you run Photoshop or some other software show creates large temp files.